26 AGRICULTURE ON THE PACIFIC SLOPE 



and makes it suitable (" available ") for their use, so 

 that it can be taken up by the roots. Other acids, 

 having the same effects, also come from the decay of 

 plants and humus in wet places, giving a brown color 

 to the water. Such brown or coffee-colored waters are 

 very common in swamps and bogs where the vine 

 maple grows, in western Oregon and Washington, as 

 well as in mountain and peat bogs everywhere. 



Sand, silt, and clay. The weathering caused by these 

 acids leaves behind, as the outcome of the decay of 

 most rocks, sand of all grades down to silt or dust, 

 and also clay. 



Sand of various grades forms the bulk of most soils. 

 The finest sands, whose separate grains can hardly be 

 felt between the fingers, are called silts. Sands and 

 most silts settle easily when stirred up in water. 



Clay is often colored yellow, orange, or red by iron 

 rust, which forms from the weathering of black or blue 

 rock minerals. 



How clay behaves. Pure clay does not settle in pure 

 water for a long time ; it may be made to settle quickly 

 by stirring into the clay water a little quicklime or salt. 



Method of separating soils. To show these facts, take 

 a small handful of soil and knead it into a "mud pie" 

 in the hand, then stir it in water to a creamy mush, and 

 pour it slowly into a tall glass (or a student-lamp chim- 

 ney closed at the small end) filled three quarters full of 

 water. The coarse sand will settle to the bottom 



